Municipal Waterborne Giardiasis: An Epidemiologic Investigation

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Abstract
In March 1976, 128 persons in Camas, Washington, USA had laboratory-confirmed giardiasis. A questionnaire survey of 498 Camas residents revealed that 3.8% had clinical giardiasis, while none of 318 residents in a control town were ill. No associations between illness and sex, pet ownership, travel, time spent in wilderness areas, public gatherings or food preference were found. Giardia cysts were recovered from raw water entering the city water treatment system via 2 streams and also from 2 storage reservoirs containing chlorinated and filtered stream water. Failure to remove Giardia cysts was attributed to the water plants'' inadequate flocculation, coagulation and sedimentation combined with deterioration of the filter media. Investigation of the watershed revealed no signs of human fecal contamination. Animal trapping in the watershed area yielded 3 beavers (Castor canadensis) infected with Giardia that were infective for specific pathogen-free beagle pups.
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